How to display a black image when screen shot,printscreen...

It is not possible. You can limit right clicks and various other methods, but sad truth is that whatever you try you cannot prevent downloading completely because you have to download images to show them. I could find them in tmp folder if nothing else.

What you CAN do is put watermark over the images to protect them from simple download. It will make it harder for the potential thief to use them. Complex watermark that is hard to remove is the best (cris-crossing lines with text in the middle with shadow or some repeating pattern for example)